1. Field of the Invention
This invention relates to polyorganosiloxane compositions containing polytetrafluoroethylene powder and to the elastomeric articles obtained from curing said compositions. More particularly, this invention relates to curable polyorganosiloxane compositions containing less than one percent by weight of polytetrafluoroethylene powder and to the cured elastomeric articles having greatly reduced porosity, fewer surface irregularities and a low durometer that are obtained by extruding and heat-curing said compositions.
2. Description of the Prior Art
Low durometer polyorganosiloxane elastomers have many useful applications. Several methods of obtaining a low durometer in polyorganosiloxane elastomers have been revealed. One method involves adding a nonfunctional low molecular weight polydimethylsiloxane fluid to the composition before curing the composition. Norbert G. Dickmann, U.S. Pat. No. 2,819,236 teaches the addition of certain fluids bearing silicon-bonded alkyl, phenyl and vinyl radicals to the uncured compositions. It is also well known to obtain a cured composition of low durometer by reducing the silica filler content in the uncured composition.
All of these known methods have shortcomings. In particular, it is well known that when the amount of silica filler in a polyorganosiloxane composition is greatly reduced the composition becomes sticky and difficult to extrude in a smooth, uniform shape. Furthermore, a curable, low-silica composition is more prone to contain small bubbles after it has been extruded and heat-cured; an undesirable result generally known as porosity.
It is known to mix polytetrafluoroethylene powder with curable polyorganosiloxane compositions to obtain better strength in the cured compositions and/or better handling properties in the uncured composition. (U.S. Pat. Nos. 2,710,290, 2,927,908, 3,132,116, 3,449,290, and Canadian Pat. No. 567,259). However, nothing has been taught regarding the use of small amounts, for example, less than 1 percent by weight, of a polytetrafluoroethylene powder in curable polyorganosiloxane compositions as a means of greatly reducing the porosity of the extruded, heat-cured composition.